The automotive industry faces an ongoing challenge of improving safety and crash-survivability of the automobiles it produces, while at the same time improving fuel efficiency to meet or exceed legislated minimum standards. One way of achieving both goals relies on the use of lighter weight materials that possess excellent mechanical strength, high impact resistance, etc. In this way the overall weight of the vehicle can be reduced, so as to achieve improved fuel efficiency, without sacrificing the capacity to absorb impact energy in the event of a collision. This strategy is widely employed to produce anti-intrusion, structural or safety components of automotive vehicles, such as for instance bumpers, door reinforcements, B-pillar reinforcements and roof reinforcements.
Oftentimes it is necessary to join together different sheet metal pieces in order to form a desired part. For instance, “butt-welded blanks” are formed by joining together, preferably by laser welding, two or more steel blanks of different compositions and/or different thicknesses. After the welded-blanks have been cold-pressed, parts are obtained having properties of mechanical strength, pressability and impact absorption that vary within the parts themselves. It is therefore possible to provide different mechanical properties at different locations within a part, without imposing an unnecessary or costly penalty on the entire part. For instance a B-pillar may be obtained by joining together a first steel blank having a high mechanical strength and a second steel blank having a relatively lower mechanical strength. During an impact, deformation is concentrated within the portion of the B-pillar that is formed from the second steel blank, such that the energy of the impact is safely absorbed in a desired fashion.
In order to avoid the need to provide a controlled furnace atmosphere during hot forming of such laser welded blanks, and also to provide improved corrosion resistance, it is common to fabricate such blanks using coated sheet metal materials, such as for instance boron steels with an aluminum-silicon pre-coating. Unfortunately, the process of laser welding such pre-coated sheet metal materials results in some of the pre-coat material being transferred into the molten area that is created during the welding operation. Subsequent austenizing and quenching of the welded blank results in the metal elements from the pre-coat material becoming alloyed with the iron or other elements of the steel sheet, thereby forming brittle, intermetallic compounds in the welded joint. On subsequent mechanical loading, these intermetallic compounds tend to be the site of onset of rupture under static or dynamic conditions. As such, the overall deformability of the welded joints after heat treatment is significantly reduced by the presence of these intermetallic compounds resulting from welding and subsequent alloying and austenizing. Another adverse effect resulting from aluminum contamination in the weld metal is the inhibition of the formation of martensite structure in a subsequent hot-stamping operation, such that the weld metal has reduced strength.
In U.S. Pat. No. 8,614,008, Canourgues et al. note that it is desirable to eliminate the source of the above-mentioned intermetallic compounds, namely the initial surface metal coating that is melted during laser welding. However, simply eliminating the pre-coated area on either side of the future weld joint, after the welding operation, results in areas on either side of the welded joint that no longer have any surface metal pre-coating. This occurs because the width of the area from which the pre-coating is removed must be at least equal to the width of the area that is melted during welding, so as not to encourage subsequent formation of intermetallic areas. Canourgues et al. note that in practice the width of pre-coat that is removed must be much more than this minimum amount to allow for fluctuations in the width of the molten area during the assembly operation. Unfortunately, during further alloying and austenizing heat treatment, scale formation and decarburizing occurs within the uncoated areas that are located next to the weld. Further, it is these uncoated and therefore unprotected areas that tend to corrode when the parts go into service.
Canourgues et al. go on to disclose their surprising discovery that eliminating only a portion of the pre-coat is still effective to solve the above-noted corrosion problem. In particular, their solution involves removing the entire thickness of the metal alloy layer while leaving in place the underlying intermetallic alloy layer that is in contact with the steel substrate. Canourgues et al. stress the precise removal of the metal alloy layer, including measuring the emissivity or reflectivity of the surface that is exposed during the removal process, and stopping the removal when a difference between the measured value and a reference value exceeds a critical threshold. Since the intermetallic alloy layer remains undisturbed during the removal of the metal alloy layer, the width of the area from which the metal alloy layer is removed may be 20-40% larger than the half width of the weld. During the welding process the metal alloy layer cannot melt into the weld pool, and as such the intermetallic areas do not form along the welded joint. The undisturbed intermetallic alloy layer on either side of the welded joint provides protection against corrosion when the part goes into service, but does not contribute significantly to the formation of intermetallic compounds in the welded joint.
The solution that is disclosed by Canourgues et al. is elegant and results in a strong weld joint that is protected against corrosion, but it is also very difficult to implement in practice. In particular, it is very difficult to achieve precise removal of the metal alloy layer by mechanical brushing or laser ablation while leaving the underlying intermetallic alloy undisturbed. Further, the process is time consuming and labor intensive, since each part of a welded blank must be handled separately, placed in a first work station to undergo removal of the metal alloy layer, moved to a second work station and positioned relative to another part of the welded blank, and then finally the separate parts are welded together in the second work station. Of course, operating separate work stations for the removal of the metal alloy layer and for the welding process increases floor-space usage requirements, and necessitates the duplication of laser sources and laser optic assemblies, etc. This is necessarily the case because a pulsed-wave laser is used to remove the metal alloy layer and a continuous-wave laser is used to perform laser welding. In particular, Canourgues et al. describe the use of a high energy-density beam, which causes vaporization and expulsion of the surface of the pre-coat.
It would be beneficial to overcome at least some of the above-mentioned limitations and disadvantages of the prior art.